For quantitative and statistical data processing on time distribution of photons, a photon correlator counts the number of photons incident on the detector during a time when the sampling gate is open (hereinafter referred to as “sampling time”), and the number of photons incident on the detector during a sampling time which is a time τ later than the previous sampling time, thereby calculating an autocorrelation coefficient.
FIG. 5 illustrates a method of counting the number of photons in which the horizontal axis indicates time t. A sampling time is indicated by ts. The number of photons detected within the sampling time at a time t is indicated by N (t), and the number of photons detected within the sampling time at a time (t+τ) is indicated by N(t+τ). The autocorrelation coefficient G(t) can be obtained by calculating the product of N(t) and N(t+τ), and then integrating with respect to time t. Generally, the above mentioned correlation time τ is in an extremely wide range from several microseconds to several dozen milliseconds.
For such autocorrelation coefficient calculation, either a photon correlator utilizing hardware or a photon correlator utilizing software has conventionally been used.
A photon correlator utilizing hardware has a mechanism for counting the number photons mentioned above, and a multiplier for cumulative multiplication, which is realized in a shift register or the like, for performing autocorrelation calculation based on the counted number of photons, and is characterized in that it is capable of performing high-speed, real-time correlation calculations.
On the other hand, a photon correlator utilizing software performs data processing by storing the number photons sampled in the memory, and reading out the count data that have been stored in the memory in accordance with a program. Accordingly, the sampling time and data processing method can be set and modified flexibly.
In the above-mentioned photon correlator utilizing hardware, the parameters for autocorrelation calculation including the sampling time ts, the available range of correlation time τ, the increment with which the correlation time τ is extended within the range, and the normalization method are preliminarily fixed. Accordingly, flexible data processing such as increasing the resolution during a correlation time in a specific range is impossible. In addition, it is also impossible to remove abruptly detected data of scattering light caused by dust in the sample.
On the other hand, the photon correlator utilizing software takes longer processing time than the photon correlator utilizing hardware. When a great deal of photon data are loaded to obtain a long correlation time, processing of the data takes such a long time that the photon measurement is suspended during the period, causing the problem of poor data loading efficiency.
It is therefore an object of the present invention to provide a photon correlator having both the high-speed of hardware processing and the flexibility of software processing.